The surface application of certain organosilicon compounds to initially untreated, chloride process-derived titanium dioxide particles has been described for allowing incorporation of the particles at high loadings, high processing rates and with a high degree of dispersion into various thermoplastic polymer matrices, particularly polyolefin derived matrices, see for example, U.S. Pat. Nos. 5,607,994; 5,631,310; 5,889,090; and 5,959,004. In addition, the treatment has been known to allow the subsequent production of finished articles, e.g., films, which are unaffected by the development of imperfections because of the release of particle associated volatiles. During high temperature thin film fabrication, these imperfections are typically referred to as lacing.
However, a problem associated with use of these surface treated particles is their inability to varying degrees to resist the UV light induced formation of chromophores (typically yellow) when the particles are incorporated into polymer matrices possessing (in concert) certain types of phenolic stabilizers (such as, for example, butylated hydroxytoluene or butylated hydroxyanisole) and hindered amine light stabilizers (such as, for example, bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate). Another problem is their inability to yield particle/polymer composites that possess any significant photodurability, that is, resistance of the particle/polymer composite to UV light induced degradation.
Substantially encapsulating the particles, before the addition of any organic surface treatment, with certain metal oxides (for example, silica, alumina, or mixtures thereof) deposited in an aqueous environment by either batch or continuous operations has been described to solve the problems of chromophore formation and photodurability, see, for example, U.S. Pat. Nos. 3,437,502; 5,993,533; and 6,783,586. However, a serious disadvantage of this solution is that the resulting metal oxide shell is prone to moisture retention and/or moisture generation which can, under high temperature fabrication conditions, result in the formation of the aforementioned polymer matrix imperfections, e.g., lacing in high temperature thin film fabrication.
It has now been found that the above-described problems and disadvantages can be significantly overcome and the highly desirable benefits associated with the aforementioned organosilane surface treatment technology retained by coupling an organosilane surface treatment with the encapsulation technology described in US Patent Publication No. 2003/0051635, incorporated herein by reference in its entirety. The encapsulation technology described in US Patent Publication No. 2003/0051635 encapsulates titanium dioxide particles with a thin shell of pyrogenically deposited silica. It has surprisingly been found that combining these techniques allows the preparation, using standard commercial production equipment, of titanium dioxide particles possessing the attributes of a high degree of polymer additive derived discolouration resistance, good photodurability, excellent volatilization resistance, high dispersibility, good processing in high load polymer matrices, excellent optical properties, and high bulk density.